[mesa-users] Low mixing length alpha values -> convergence problems

[Aaron Dotter]

Hi Lewis,

Setting aside the issue of why you want to use alpha_MLT=0.01...

Where in the run does it have convergence problems?  Near the beginning or
sometime later?

Because you can create a saved model that has evolved down the Hayashi line
for a while using a "normal" alpha_MLT value and then start from that
instead of using create_pre_main_sequence_model.  See
mesa/star/defaults/star_job.defaults:

      save_model_number = -111

         !### save_model_when_terminate
         ! save final model when a run terminates

      save_model_when_terminate = .false.

         !### save_model_filename
         ! saved model root filename

      save_model_filename = 'undefined'

and

      load_saved_model = .false.
      saved_model_name = 'undefined'

No guarantee that this will solve your problem but it is worth a try.


Aaron


On Wed, Aug 19, 2015 at 8:11 PM, Lewis Ireland <lgi201 at exeter.ac.uk> wrote:

> Hello,
>
> I am using the work directory (tutorial folder) to create 0.5 solar mass
> models, all at different mixing length parameters (to see the change in R
> when it nears ZAMS). I have these set:
>
> *! begin with a pre-main sequence model*
> *    create_pre_main_sequence_model = .true.*
>
> *  ! starting specifications*
> *    initial_mass = 0.5d0 ! in Msun units*
>
> *mixing_length_alpha = 0.55d0*
>
> I have the following set to end the run:
>
> *  ! stop when the star nears ZAMS (Lnuc/L > 0.99)*
> *    Lnuc_div_L_zams_limit = 0.99d0*
> *    stop_near_zams = .true.*
>
> *  ! stop when the center mass fraction of h1 drops below this limit*
> *    xa_central_lower_limit_species(1) = 'h1'*
> *    xa_central_lower_limit(1) = 1d-3*
>
>  I need to go to real low values of mixing length parameter, e.g. 0.1,
> 0.01 etc., but when I go to or below alpha = 0.5, I get:
>
> hydro_newton_step failed to converge
>        retry cnt, step, log10(dt/yr), retry_factor     1   543
> -14.045150        0.500000
>                                                      dt
> 5.6884209599392411D-07
>                                      min_timestep_limit
> 9.9999999999999995D-07
>
>  stopping because of convergence problems dt < min_timestep_limit
>
>
> terminated evolution: convergence problems
>
> termination code: min_timestep_limit
>
> How can I go about using lower values of alpha without this problem? Is
> there another test_suite you recommend me to use that would benefit my
> models or get past this convergence issue?
>
> Cheers,
>
> Lewis
>
>
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